EXCHANGE 


unfit  'iz  Nvr  - 


THE  EXTENSION  OF  THE  X-RAY 

SPECTRUM  TO  THE 

ULTRAVIOLET 


orr  4 


A  DISSERTATION 

PRESENTED  TO  THE  FACULTY  OF  PRINCETON  UNIVERSITY  IN 
CANDIDACY  FOR  THE  DEGREE  OF  DOCTOR  OF  SCIENCE 


BY 


EDWARD  H.  KURTH 


Reprinted  from  THE  PHYSICAL  REVIEW,  N.  S.,'  Vol.  XVIII. ,  No.  6,  December,  1921. 


THE  EXTENSION  OF  THE  X-RAY 

SPECTRUM  TO  THE 

ULTRAVIOLET 


A  DISSERTATION 

PRESENTED  TO  THE  FACULTY  OF  PRINCETON  UNIVERSITY  IN 
CANDIDACY  FOR  THE  DEGREE  OF  DOCTOR  OF  SCIENCE 


BY 
EDWARD  H.    KURTH 


Reprinted  from  THE  PHYSICAL  REVIEW,  N.  S.,  Vol.  XVIII.,  No.  6,  December,  1921. 

• 


ACCEPTED  BY  THE 

APR  1922 
DEFT.  OF  PHYSICS 


[Reprinted  from  THE  PHYSICAL  REVIEW,  N.S..  Vol.  XVIII,  No.  6,  December,  1921.] 


THE   EXTENSION   OF  THE   X-RAY   SPECTRUM   TO    THE 

ULTRAVIOLET. 

BY  E.  H.  KURTH. 
SYNOPSIS. 

Characteristic  X-Radiation  Due  to  Slow  Electrons,  1,000  to  12  Volts. — By  using 
the  methods  of  modern  high-vacuum  technique,  the  difficulties  experienced  by 
previous  investigators  have  been  largely  overcome.  The  radiation  was  measured 
in  terms  of  the  photo-electric  current  excited  from  a  Pt  dish.  The  effect  was  made 
large  by  utilizing  a  hot  tungsten  helix  as  a  source  of  electrons,  and  all  disturbances 
due  to  gas  ions  formed  by  the  electrons  were  eliminated  by  maintaining  a  very  high 
vacuum  and  by  interposing  an  electrostatic  field  across  the  path  of  the  radiation. 
The  deflections  thus  obtained  were  large  and  perfectly  reproducible.  When  the 
deflections  per  unit  thermionic  current  were  plotted  as  a  function  of  the  accelerating 
potential,  sharp  breaks  appeared  which  each  indicate  the  minimum  energy  necessary 
to  excite  the  corresponding  characteristic  radiation.  From  these  energy  deter- 
minations the  corresponding  wave-lengths  were  computed  using  the  quantum 
relation.  Thus  the  following  values  have  been  obtained  for  the  convergence  wave- 
lenghts  in  Angstroms:  K-series  of  car  bo  n  42.6,  oxygen  23.8;  L-series  of  carbon  375, 
oxygen  248,  aluminum  ioof. silicon  82.5,  titanium  24.5,  iron  16.3,  copper  12.3;  M- 
series  of  aluminium  326,  titanium  85.3,  iron  54.3,  copper  41.6;  N-series  of  iron  247, 
copper  116.  The  relation  of  these  results  to  those  obtained  by  crystal  analysis  and 
by  spectrum  analysis  is  discussed.  It  is  suggested  that  the  radiation  from  solid 
targets  may  differ  from  the  radiation  from  gaseous  atoms,  especially  for  the  lighter 
elements. 

TT  was  probably  first  suggested  by  Sir  J.  J.  Thomson  that  it  might  be 
-*•  practicable  to  investigate  by  means  of  x-radiation  produced  at 
relatively  low  voltages  that  region  of  the  spectrum  falling  between  the 
wave-length  of  the  longest  measured  x-rays  and  that  of  the  shortest 
ultraviolet  radiation  studied  spectroscopically.  Difficulties  in  ruling 
suitable  gratings  and  their  low  reflecting  power  have  limited  the  explora- 
tion of  this  region  from  the  ultraviolet  side,  while  the  close  grating  space 
of  crystals  and  their  strong  absorption  of  the  radiation  have  prevented 
the  application  of  the  methods  of  x-ray  analysis.  Now  from  the  quan- 
tum relation  eV  =  hv,  it  is  evident  that  the  upper  frequency  limit  of 
this  region  corresponds  to  electronic  impacts  of  approximately  1,000 
volts.  Investigators  thus  far,  however,  have  been  chiefly  concerned 
with  the  question  of  the  actual  production  of  radiation  by  impacts  of 
these  slow-moving  electrons  against  a  solid  anticathode,  and  experiments 
have  indicated  its  presence  for  potentials  down  to  values  below  100 
volts.  Convincing  evidence,  however,  of  the  presence  of  radiation  in 


464 


E.   H.    KURTH. 


[SECOND 

[SERIES. 


wire  was  mounted  in  the  small  end  of  a  large  conical  glass  tube.  The 
target,  T,  normally  about  6  by  12  mm.  in  section,  was  arranged  to  be 
changed  at  will  through  the  agency  of  a  small  ground  glass  joint,  /, 
which  was  made  tight  with  a  little  De  Kotinsky  cement.  The  long 
glass  stem  on  which  the  target  is  mounted  was  designed  to  fit  the  side 
tube  quite  closely,  and  originally  a  short  glass  appendix  which  could  be 
immersed  in  liquid  air  was  attached  at  the  base  of  the  seal.  This  device, 
it  was  hoped,  would  make  it  improbable  that  any  trouble  might  be 
encountered  due  to  the  leakage  into  the  apparatus  of  water  vapor  coming 
off  the  glass  near  the  joint.  In  actual  operation,  however,  it  was  found 
that  the  escape  of  gas  from  this  joint  was  negligibly  small.  In  the  body 

of  the  tube  a  system  of  seven  diverg- 
ing plates  was  arranged,  between  which 
the  radiation  from  the  target  must  pass 
before  reaching  the  further  compart- 
ment of  the  tube.  Alternate  plates  of 
the  set  were  connected  together  form- 
ing two  groups,  P  and  P',  one  of 
which  was  then  joined  to  the  gauze  G 
and  the  other  to  the  gauze  G' .  Leads 
from  these  two  gauzes  as  well  as  from 
the  third  gauze,  G" ,  were  brought  to 
the  outside  of  the  tube  as  indicated  in 
the  figure.  These  gauzes  are  all  of 
exceptionally  coarse  mesh  and  they  do 
not  intercept  an  appreciable  propor- 
tion of  the  radiation.  The  radiation 
detecting  plate,  D,  is  about  5  cm.  in 
diameter  and  is  made  of  platinum,  as 
are  all  of  the  other  metal  parts.  Ap- 
propriate guard  ring  devices,  not  shown 


o  fctfc 


Fig.  1. 


in  the  drawing,  were  provided  to  take  care  of  volume  and  surface  leakage 
of  electricity  to  the  detecting  disk. 

A  large  two-stage  condensation  pump  backed  up  by  an  oil  pump  is 
used  to  evacuate  the  apparatus.  Large  bore  tubing  is  employed  for 
connections  and  two  liquid  air  traps,  one  of  which  contained  charcoal, 
are  located  between  the  pump  and  the  apparatus  for  the  purpose  of 
freezing  out  mercury  vapor.  A  reasonably  sensitive  McLeod  gauge  and 
a  mercury  cut-off  are  provided  close  to  the  pump  for  use  in  detecting 
the  presence  of  possible  leaks.  An  appendix  containing  a  small  quantity 
of  charcoal  is  also  connected  directly  to  the  apparatus.  Electric  heaters 


THE   EXTENSION   OF    THE   X-RAY   SPECTRUM.  465 

are  arranged  to  bake  out  the  apparatus,  charcoal  and  traps  at  about  400° 
for  several  hours  before  every  run.  Simultaneously  the  connecting  tube 
is  well  heated  with  a  Bunsen  flame.  Finally  at  the  conclusion  of  this 
heating  process  the  charcoal  and  vapor  traps  are  immersed  in  liquid  air. 

As  long  as  the  charcoal  is  being  heated,  a  pressure  of  a  small  fraction 
of  a  bar  is  always  indicated  by  the  gauge.  When  this  heating  is  dis- 
continued, however,  the  pressure  at  once  becomes  immeasurably  small. 
The  pump  is  always  kept  in  operation  during  the  runs. 

The  tungsten  cathode  is  heated  by  a  set  of  high-capacity  storage  cells. 
Its  resistance  is  approximately  0.7  ohm  and  it  requires  from  4  to  6 
amperes  to  light  it.  The  negative  end  of  the  cathode  is  joined  to  the 
gauze  G  and  the  connection  is  earthed.  The  anode  voltage  is  provided 
by  a  small  1 ,500- volt  direct-current  generator.  This  potential  is  regulated 
by  slide  resistances  and  is  measured  on  a  150- volt  Weston  voltmeter 
which  is  fitted  with  an  adjustable  series  resistance  to  give  suitable  range 
to  the  scale.  The  thermionic  current  in  this  circuit  is  read  upon  a  Paul 
Universal  Testing  Set.  The  set  of  plates  which  is  joined  to  the  gauze 
G'  is  connected  to  a  group  of  dry  cells  which  provide  an  adjustable 
source  of  potential  up  to  600  volts.  The  gauze  G" ,  which  receives  the 
photo-electrons  from  the  detecting  disk,  D,  is  raised  to  a  potential  of  35 
volts  furnished  by  a  small  battery.  A  Dolezalek  electrometer  with  a 
sensitivity  of  about  1,700  mm.  per  volt  is  connected  to  the  detecting  disk, 
and  the  instrument  is  fitted  with  a  series  of  India  ink  shunts  of  different 
resistances.  Thus  definite  scale  deflections  rather  than  rate  of  deflection 
are  observed  when  radiation  falls  upon  the  detecting  disk. 

In  the  original  set  up  a  large  electro-magnet  was  arranged  so  that  a 
strong  magnetic  field  could  be  applied  perpendicularly  to  the  plates. 
Under  the  influence  of  this  field  the  normal,  direct  path  of  an  ion  passing 
between  the  plates  would  become  converted  into  a  series  of  loops,  and 
the  corresponding  length  of  time  that  the  ion  would  remain  between  the 
plates  would  be  considerably  increased.  One  might  expect  to  remove  the 
ions  under  these  conditions  with  an  electric  field  of  very  moderate 
strength  between  the  plates.  When  the  arrangement  was  actually  tried, 
however,  serious  complications  were  introduced.  The  stray  magnetic 
field  greatly  reduced  the  thermionic  bombarding  current  and  the  photo- 
electric current  from  the  disk.  The  tube  was  therefore  carefully  shielded 
from  the  effects  of  the  stray  lines  by  means  of  a  series  of  soft  steel  frames. 
When  this  was  done,  and  when  an  electric  field  of  about  100  volts  was 
applied  to  the  plates,  it  was  found  that  variations  in  the  strength  of  the 
magnetic  field  did  not  affect  the  magnitude  of  the  electrometer  deflec- 
tions. As  a  result  of  this  test  the  magnetic  field  was  proved  unnecessary 


466  E.    H.    KURTH. 

and  this  feature  was  completely  eliminated  after  a  few  of  the  preliminary 
runs  on  the  apparatus. 

In  fact  it  is  easy  to  show  by  a  simple  calculation  that  it  ought  to  be 
possible  to  remove  ions,  moving  with  the  maximum  velocity  that  one 
might  reasonably  expect  under  the  conditions,  by  the  application  of  a  very 
moderate  potential  to  such  a  system  of  plates.  The  calculation  shows 
that  a  hydrogen  ion,  for  instance,  with  a  velocity  corresponding  to  a  fall 
through  1,000  volts,  would  be  removed  by  a  plate  potential  of  the  order 
of  100  volts.  Secondary  effects  arising  from  the  impacts  of  the  ions 
against  the  plates  in  this  apparatus,  furthermore,  are  not  likely  to  be 
serious  if  the  number  of  impacts  is  not  excessive.  With  a  view,  therefore, 
to  reducing  the  number  of  ions  present  to  a  minimum  particular  care 
has  been  exercised  to  secure  the  very  best  vacuum  conditions,  and,  as 
far  as  possible,  to  free  the  target  from  occluded  gas. 

Thus,  preliminary  to  all  runs,  the  target  is  given  a  thorough  heat 
treatment,  which  consists  in  raising  it  to  as  high  a  temperature,  by 
electronic  bombardment  from  the  cathode,  as  the  particular  element  in 
use  will  safely  withstand.  A  current  of  about  30  milliamperes  at  300 
volts  is  sufficient  to  bring  the  anode  to  a  bright  red  heat.  This  pre- 
liminary heat  treatment  of  the  target  is  always  carried  out  at  a  higher 
temperature  than  will  be  reached  in  the  subsequent  run. 

EXPERIMENTAL  TESTS. 

When  the  cathode  is  heated,  a  small  deflection  of  the  electrometer 
regularly  takes  place  before  the  anode  voltage  is  applied  and  it  is  in  a 
direction  corresponding  to  a  positive  charging  up  of  the  detecting  disk. 
This  deflection  results  from  a  photoelectric  action  upon  the  disk  pro- 
duced by  intercepted  light  from  the  glowing  cathode.  Its  magnitude 
depends  upon  the  temperature  of  the  cathode,  and  when  this  is  very  high, 
it  may  amount  to  25  millimeters  with  a  shunt  permitting  moderate 
electrometer  sensitivity.  This  small  deflection  is,  of  course,  constant 
for  any  particular  set  of  readings,  and  its  effect  is  completely  eliminated 
by  resetting  the  zero  of  the  scale. 

If  now  the  anode  voltage  be  gradually  applied — assuming  a  potential 
of,  say,  100  volts  between  the  plates — the  electrometer  will  remain  un- 
affected until  a  potential  of  from  12  to  25  volts  is  attained  depending  on 
the  material  of  the  target.  Then  it  will  begin  to  deflect  slowly  in  the 
same  direction  as  before,  and  the  magnitude  of  this  deflection  increases 
rapidly  with  further  increase  of  the  anode  voltage. 

If  the  anode  voltage  be  now  adjusted  to  some  value  which  will  give 
a  fair  scale  deflection,  an  increase  in  the  potential  across  the  plates  will 


THE   EXTENSION   OF    THE   X-RAY    SPECTRUM.  467 

not  change  its  magnitude.  If,  however,  the  plate  potential  be  reduced, 
the  deflection  will  not  change  until  a  comparatively  low  critical  potential 
difference  is  attained.  Then  a  sudden  increase  is  observed,  and  a  further 
decrease  in  the  plate  potential  will  result  in  an  off-scale  deflection.  The 
setting  in  of  this  effect  is  certain  indication  that  positive  ions  formed  in 
the  radiation  compartment  are  beginning  to  pass  between  the  plates  to 
the  detecting  disk.  The  value  of  this  critical  plate  potential  seems  to 
be  somewhat  characteristic  of  the  target  element  which  is  being  used. 
With  the  carbon  target  it  was  found  to  be  as  high  as  30  volts  but  with 
titanium  it  was  below  12  volts.  Since,  during  most  of  the  actual  runs, 
the  plates  have  been  charged  to  135  volts,  these  experiments  indicate 
unquestionably  that  the  system  of  plates  as  used  is  perfectly  effective  in 
preventing  the  passage  of  charged  bodies  through  to  the  detecting  disk. 
There  is  still,  however,  the  question  of  the  radiation  associated  with 
the  formation  of  these  ions  to  be  considered.  No  direct  tests  with  a 
view  to  differentiating  between  the  true  target  radiation  and  gas  radiation 
have  yet  been  made.  However,  there  are  several  indications  that  the 
latter  effect  may  be  safely  neglected  in  the  present  investigation.  First, 
the  results  secured  are  characteristic,  in  a  recognizable  fashion,  of  the 
different  elements  used  as  targets.  There  is  no  reason  to  believe  that 
radiation  from  residual  gas  would  behave  in  this  manner.  Second,  the 
only  gas  presumedly  present  which  might  cause  trouble  in  this  work  is 
oxygen.  No  characteristic  radiation  effects  from  this  gas  similar  to 
those  which  were  later  secured  when  the  element  itself  was  used  in  an 
oxide  as  a  target  were  ever  observed.  Third,  the  gas  pressure  was 
known  to  be  too  low  to  permit  a  sufficient  number  of  impacts  against 
gas  atoms  to  give  a  detectable  effect. 

RESULTS. 

When  the  electrometer  deflection  is  plotted  as  a  function  of  the  anode 
potential,  curves  of  the  type  shown  in  Fig.  2  are  obtained.  In  some 
cases  marked  discontinuities  or  "breaks"  in  the  curvature  are  dis- 
cernible, as  in  the  curve  for  iron  referred  to,  while  in  others  it  is  practically 
impossible  to  determine  with  any  degree  of  precision  the  position  of  a 
definite  break  in  the  curve.  If,  however,  the  deflection  per  unit  ther- 
mionic current  is  plotted  against  the  anode  potential,  the  resulting  relation 
is  linear.  Any  change  in  the  rate  of  increase  of  the  effect  with  the 
voltage  will  now  be  very  evident,  and  to  determine  the  position  of  the 
break  point  with  considerable  precision,  one  has  only  to  draw  the  two 
best  straight  lines  through  the  plotted  points  on  each  side  of  the  break, 
and  the  position  of  the  break  will  be  indicated  by  their  point  of  inter- 
section. 


468 


E.   H.    KURTH. 


F SECOND 
[SERIES. 


N;  Sertes: 


The  radiation  produced  by  impacts  of  electrons  against  a  solid  consists 
of  two  distinct  types:  general  radiation  and  characteristic  radiation. 
Both  types  are  generally  present  and  both  evidently  increase  in  intensity 
linearly  with  the  voltage.  A  break  point  in  the  curve  indicates  the  setting 
in  of  a  new  type  of  radiation,  which  is  found  to  be  characteristic  of  the 
target  element.  From  a  priori  considerations  the  break  may  be  either 
an  upward  or  downward  inflection,  depending  on  whether  or  not  the  new 
characteristic  radiation  produces  a  larger  photoelectric  effect  than  would 

be  produced  by  the  additional  general 
radiation  which  would  be  emitted  if 
this  characteristic  radiation  did  not 
set  in.  It  has  been  found  that  in 
general  the  setting  in  of  characteristic 
radiation  is  indicated  by  an  increase  in 
the  total  emission.  However,  one  ex- 
ception has  been  thus  far  noted  in 
these  experiments.  The  L  series  of 
silicon  is  evident  as  an  actual  falling 
off  in  the  total  radiation.  Evidently, 
in  this  case,  the  increase  in  character- 
istic radiation  is  insufficient  in  amount 
to  balance  the  falling  off  in  general 
radiation.  This  phenomenon  has 
been  previously  noted  in  an  investi- 
gation of  the  variation  of  total  x-ray 
intensity  with  voltage  in  the  case  of  silver,  for  radiations  of  the  ordi- 
nary x-ray  type.1  However,  while  the  presence  of  characteristic  radia- 
tion is  usually  shown  by  an  upward  inflection  in  the  curve,  the 
sharpness  of  the  break  varies  considerably  with  the  different  elements 
and  among  the  several  x-ray  series  of  the  same  element.  It  is  hoped 
that  later,  when  more  elements  have  been  studied,  it  will  be  possible 
to  establish  some  sort  of  a  periodic  relationship  relative  to  the  inten- 
sity of  the  characteristic  radiation  from  the  different  elements. 

CARBON. 

The  radiation  curves  for  carbon  are  given  in  Fig.  3.  The  upper  curve 
shows  the  break  corresponding  to  the  K  series  of  the  element  and  the 
lower  one  indicates  the  L  series.  Both  curves  are  the  results  of  single 
runs  over  the  respective  ranges.  The  target  in  this  case  was  cut  from 
a  piece  of  graphite,  and  during  the  procedure  of  evacuating  the  appa- 
ratus, it  was  brought  to  a  white  heat  by  thermionic  bombardment. 

1  Brainin,  PHYS.  REV.,  10,  p.  461,  1917. 


NoL'6XVIIL]  THE   EXTENSION   OF    THE    X-RAY   SPECTRUM.  469 

Despite  this  exceptionally  favorable  heat  treatment,  however,  the  carbon 
seemed  to  require,  in  order  to  eliminate  the  direct  effect  of  positive  ions, 
a  little  higher  minimum  voltage  across  the  plate  than  any  other  element 
thus  far  studied.  Perhaps  this  fact  may  account  for  the  apparent  slight 
departure  which  is  to  be  noted  in  the  case  of  the  carbon  curves  from  a 
fair  linear  relationship  between  break  points.  The  L  series  break  indi- 
cated for  this  element  is  of  special  interest  because  it  presumedly  arises 
as  a  result  of  electrons  falling  into  the  very  outer  shell  of  the  atom.  There 
was  considerable  reason  to  expect,  in  fact,  that  one  might  actually  not 
observe  a  sharp  break  corre- 


.  .  «     «  •  r  '::-  —p  — -'.'--; •"--'-""!"• -:j;'iH---'^ *"i" ''1 --' •  •; • : If: vttdrM rnutfrtlj 'H^TJ^ftJTffi^B 

spending  to  this  series  for  car- 
bon in  the  solid  form  because 
of  the  proximity  of  the  neigh-      7H 
boring  atoms.     As  it  is,  there     ^\ 
is  probably  little  question  but       90|f 
that  the  potential  energies  of       80|iflj 
the  electrons  in  the  outer  shell       70 ;" 
of    the    carbon    atom    vary       w*~- 
somewhat,    depending    upon       so '•)«-- 
whether  the   atom   is    free  as       ,  .,-.     .•' 
in  a  vapor  or  whether  it  is 
combined  with  other   atoms 
as     in    a    solid.     Thus     one 

might  expect  to  find  that  the  L  series  of  the  solid  carbon,  for  instance, 
is  somewhat  different  from  the  L  series,  if  it  might  be  obtained,  of  the 
vapor. 

COPPER. 

The  copper  target  used  in  these  experiments  was  heated  for  a  consider- 
able time  to  a  temperature  close  to  its  melting  point.  Finally,  just  prior 
to  making  the  runs,  the  temperature  was  raised  until  the  target  melted 
a  little  at  one  end,  and  during  this  process,  a  considerable  quantity  of 
copper  was  distilled  upon  the  inner  surface  of  the  glass  tube.  In  the 
case  of  one  of  the  preliminary  runs  which  was  made  with  an  unusually 
high  thermionic  current,  a  strong  break  in  the  curve  was  obtained  at 
about  500  volts.  The  position  of  this  break  was,  however,  found  to 
depend  upon  the  temperature  of  the  target  and  it  was  unquestionably 
due  to  vaporization  of  the  copper  at  these  voltages,  the  new  radiation 
arising  from  the  copper  vapor.  But  the  actual  runs,  for  which  the 
curves  are  given,  were  made  with  the  target  at  relatively  low  tempera- 
tures and  in  no  case  did  this  exceed  that  of  a  dull  red  heat. 


470 


E.   H.    KURTH. 


[SECOND 
LSERIES. 


The  results  for  copper  are  given  by  the  curves  of  Figs.  4,  5,  and  6. 
A  range  up  to  600  volts  is  shown  in  Fig.  4.     This  curve  shows  a  break 

corresponding  to  the  M  series 
and  the  presence  of  a  some- 
what less  sharply  defined  but 
intense  effect  beginning  a 
little  above  100  volts.  This 
lower  effect  is  shown  to 
better  advantage  by  the 
curve  of  Fig.  5,  taken  with 
larger  currents.  The  absence 
of  the  customary  sharpness 
in  the  case  of  this  break 
may  be  accounted  for  by  as- 
suming that  the  group  of 
lines  in  the  spectral  series 
which  it  represents  may  ac- 
tually include  two  or  more  convergence  limits.  This  is  found  to  be  the 
case  in  the  portion  of  the  M  series  already  studied  and  presumedly  it 
would  likewise  be  true  of  a  still  more  complicated  N  series.  The  L 
series  break  for  the  element  is  given  in  the  curve  of  Fig.  6. 


Volts 


Fig.  4. 


Volts 


LOO  Z40 


Fig.  5. 


OXYGEN. 

When  the  experiments  with  copper  were  concluded,  the  target  was 
removed,  and  the  surface  of  the  metal  was  carefully  oxidized  in  a  Bunsen 


VOL.  XVIII.l 
No.  6. 


THE    EXTENSION    OF    THE    X-RAY    SPECTRUM. 


471 


flame.  It  was  immediately  reinserted  and  the  apparatus  re-exhausted. 
It  was  found  that  the  oxide  on  the  copper  would  safely  withstand  a 
temperature  corresponding  to  a  low  red  heat.  The  curves  of  Fig.  7 
give  the  positions  of  the  K  and  L  series  breaks  obtained  for  this  element. 


Volts 


Fig.  6. 

The  breaks  for  copper  could  also  be  obtained  with  this  target,  'but  their 
positions  were  so  far  removed  from  the  oxygen  breaks  as  to  cause  no 
uncertainty  in  the  interpretation  of  the  results. 


wt, 


600      K        BSD 


Fig.  7. 


ALUMINIUM. 

Some  unique  difficulties  were  experienced  in  the  work  with  this  element. 
In  the  first  place,  the  bombardment  process  had  to  be  very  carefully 
carried  out  because  of  the  comparatively  low  melting  point  of  657°. 


472 


E.   H.    KURTH. 


[SECOND 

[SERIES. 


Secondly,  it  was  found  that  the  break  points  would  gradually  decrease  in 
sharpness  during  a  series  of  runs  on  successive  days  and,  eventually,  one 
would  obtain  practically  a  straight-line  relationship  over  the  entire 
region  of  the  breaks.  If  the  target  were  now  removed  from  the  appa- 
ratus, resurfaced,  and  again  replaced,  the  break  points  would  not  be 
observed,  or  at  best,  perhaps,  very  weakly,  upon  making  a  run.  In 
order  to  obtain  the  break  as  originally  it  was  necessary  to  insert  a  new 
target.  It  is  possible  that  the  unusual  behavior  of  this  element  is  due 
to  some  peculiar  action  of  mercury  vapor  upon  the  aluminium.  Since  it 
was  not  ordinarily  convenient  nor  consideied  desirable  to  keep  liquid  air 
on  the  traps  over  night  or  during  the  baking-out  treatment,  the  target 
was  exposed  to  the  action  of  the  vapor  at  three  or  four  microns'  pressure 
between  the  experiments.  However,  there  were  no  visible  indications 
of  any  effect  of  the  mercury  upon  the  target  and  if  the  tendency  were  to 
form  an  alloy,  it  is  a  little  difficult  to  see  why  a  temperature  of  red  heat 
did  not  break  it  down. 

IRON. 

The  L  and  M  series  breaks  for  iron  are  given  in  the  two  curves  of 
Fig.  8.     The  lower  break,  corresponding  to  an  N  series,  is  shown  in  the 


'Volts 


curve  of  Fig.  2.  This  break  is  likewise  evident  in  the  lower  curve  of 
Fig.  8.  The  lack  of  sharpness  of  the  M  series  break  which  is  to  be  noted 
is  evidence  that  the  M  series  at  this  point  contains  more  than  one  con- 
vergence limit. 


VOL.  XVIII.1 
No.  6. 


THE   EXTENSION   OF    THE   X-RAY   SPECTRUM. 


473 


TITANIUM. 

There  is  little  of  special  interest  evident  as  yet  with  regard  to  the 
radiation  of  this  element  except  that  the  break  corresponding  to  the  L 
series  is  exceptionally  strong.  Both  the  L  and  M  series  are  observed. 

SILICON. 

The  work  with  silicon  has  not  yet  been  completed,  but  the  tendency, 
first  noted  with  this  element,  for  the  intensity  of  the  effect  to  fall  off 
coincidently  with  the  setting  in  of  the  characteristic  radiation  has  been 

previously  discussed. 

INTERPRETATION. 

Fig.  9  shows  the  Moseley  curves  extended  to  include  the  types  of 
characteristic  radiation  discovered  in  the  investigation  described  above. 


0  V    LM 


30    J 


Fig.  9. 

The  square  roots  of  the  frequencies  of  the  characteristic  radiations  are 
plotted  against  the  atomic  numbers  of  the  chemical  elements,  showing  the 
K,  L  and  M  series  of  radiations.  The  solid  curves  refer  to  radiations 
investigated  by  the  ordinary  crystal  method  of  x-ray  analysis  and  show 
how  far  toward  the  region  of  longer  wave-lengths  each  type  of  radiation 
has  been  detected.  The  plotted  points  represent  the  results  of  the 


474  E.   H.   KURTH. 

present  investigation  and  the  dotted  lines  through  them  indicate  their 
probable  relationship  with  the  ordinary  types  of  x-radiation. 

The  abscissae  marked  V,  L  and  M  indicate  the  short  wave-length 
limits  reached  by  spectroscopic  methods  in  the  regions  designated  as  the 
ultraviolet  region,  the  Lyman  region  and  the  Millikan  region,  respec- 

o  o 

tively.  There  remains  a  large  gap  from  about  100  A.  to  about  10  A. 
in  which  no  previous  method  of  spectrosopy  has  been  applicable  and  in 
which  the  first  definite  results  are  given  by  the  present  work. 

In  conformity  with  the  results  of  Webster's  work  on  the  excitation  of 
x-rays,1  it  is  evident,  by  extrapolation  from  the  x-ray  curves,  that  the 
characteristic  radiations  in  the  present  experiments  are  excited  only 
when  the  bombarding  electrons  possess  energy  corresponding  to  the 
convergence  frequencies  of  the  series.  These  convergence  frequencies 
are  almost  identical  with  the  7  lines  of  the  K  and  L  series,  but  lie  very 
slightly  to  the  right  of  these  lines  in  Fig.  9.  In  the  K  and  L  series  the 
observed  points  fall  on  a  reasonable  extrapolation  of  the  known  lines. 
In  the  case  of  the  M  series  the  extrapolation  is  too  great  to  be  considered 
as  anything  but  a  suggestion,  whereas  the  N  series  is  purely  hypothetical, 
since  the  two  points  ascribed  to  it  may  prove  to  belong  to  a  second  group 
in  the  M  series.  It  is  planned  to  investigate  a  number  of  elements  of 
higher  atomic  number  in  order  to  connect  these  observations  with  the 
known  points  of  the  M  series. 

It  appears  that  the  K  series  continues  uniformly  down  to  hydrogen, 
for  which  the  plotted  value  is  obtained  from  the  convergence  frequency 
of  the  Lyman  series.  The  curve  is  slightly  concave  downwards. 

By  applying  the  combination  principle  it  is  possible  to  predict  the 
extension  of  the  L  a  line  as  far  as  sodium,  atomic  number  n,  and  the 
L  7  line  to  calcium,  atomic  number  20.  The  observed  frequencies  of 
the  L  radiations  of  copper,  iron,  titanium,  silicon  and  aluminium  are  all 
larger  than  the  values  predicted  by  the  combination  principle.  But  the 
observed  values  of  frequencies  in  the  ordinary  x-ray  region  are  also 
larger  than  those  predicted  by  the  combination  principle,  and  by  about 
the  same  amount.  Thus  there  is  no  reason  for  believing  that  the  curve 
through  the  observed  points  is  not  an  accurate  continuation  of  the  curve 
through  points  representing  the  L  convergence  frequencies  in  the  ordinary 
x-ray  region.  It  is  planned  to  test  some  elements  of  atomic  numbers 
slightly  higher  than  that  of  copper  in  order  to  secure  an  actual  over- 
lapping of  the  two  methods  ir  this  part  of  the  spectrum. 

The  lower  part  of  the  dotted  curve  for  the  L  series  departs  consider- 
ably from  the  extension  of  the  L  series  curves  piedicted  by  the  combina- 

1  Webster,  PHYS.  REV.,  7,  p.  599,  1916;  Webster  and  Clark,  Nat.  Acad.  Proc.,  3,  p.  185, 
1917. 


THE    EXTENSION    OF    THE    X-RAY    SPECTRUM.  475 

tion  principle,  which  principle  suggests  that  the  L  a  line  should  continue 
straight  into  the  "Y"  axis  at  about  atomic  number  7,  while  the  L  y 
line  should  lie  below  it  and  be  somewhat  concave  downwards.  Some 
recent  unpublished  results,  kindly  communicated  to  us  by  Dr.  Foote 
and  Dr.  Mohler,  on  soft  x-rays  from  gases,  fall  consistently  on,  or  to 
the  left  of,  the  extension  of  the  L  a  line  predicted  by  the  combination 
principle.  But  the  present  results  should  lie  to  the  right  of  this  line 
because  they  give  convergence  frequencies,  and  because  this  line  is  known 
to  be  to  the  left  of  the  actual  values  where  they  are  known  in  the  x-ray 
region.  Furthermore,  Millikan  has  observed  spectroscopically  the  L 
series  of  carbon,  and  Sommerfeld  points  out  strong  reasons  for  believing 
that  hydrogen  possesses  an  L  series  in  its  Balmer  series.  Thus  the 
L  curves  must  actually  curve  downward  toward  the  origin,  and  there  is 
no  obvious  reason  for  believing  that  the  L  curve  in  Fig.  9  is  not  correct. 
It  is  likely  that  the  difference  between  the  results  shown  in  Fig.  9,  which 
applies  throughout  to  radiation  from  solid  targets,  and  those  obtained 
by  Foote  and  Mohler  for  gases  and  vapors  is  due  to  an  actual  modifica- 
tion of  the  characteristic  frequencies  in  atoms  of  solids,  arising  from  the 
influence  of  neighboring  atoms.  This  modification  would  be  expected 
to  be  less  important  at  the  higher  frequencies,  but  would  probably  be 
very  important  in  the  case  of  radiation  from  electrons  in  the  outer  shells, 
or  orbits.  It  may  be  that  this  influence  accounts  for  the  inexactness  of 
Kossel's  relations  when  applied  to  x-radiation  from  solid  targets. 

The  M  a  curve  can  be  predicted  by  Kossel's  relation  down  as  far  as 
calcium.  It  shows  a  strong  curvature  downward,  occurring  at  about 
cobalt,  atomic  number  27,  and  by  analogy,  leading  us  to  expect  a  similar 
downward  inflection  near  the  foot  of  the  L  curves.  There  are  no  data 
whereby  the  convergence  frequencies  of  the  M  series  can  be  predicted 
in  the  region  in  which  we  are  interested.  The  observed  characteristic 
radiations,  ascribed  to  theM  series,  are  of  considerably  higher  frequencies 
than  the  predicted  M  a  radiation,  the  frequency  difference  being  about 
the  same  as  that  found  in  the  L  series. 

The  lowest  voltage  at  which  detectable  radiation  was  produced  is 
12.5  volts,  in  the  case  of  oxygen.  This  corresponds  to  a  wave-length 
of  990  A.  In  most  of  the  other  cases  the  radiation  was  first  detected  at 
about  20  volts.  The  relatively  large  importance  of  velocity  distribution 
corrections  and  of  slight  zero  shifts  on  the  accuracy  in  this  region  of 
low  voltages  probably  renders  these  results  of  little  interest,  except  in 
that  they  prove  the  production  of  radiation  by  impacts  at  these  small 
voltages.  It  is  hoped  that  a  subsequent  re-design  of  the  apparatus  may 
enable  better  accuracy  to  be  secured  in  this  region  of  weak  effects. 


476 


E.    H.    KURTH. 


fSlCOND 

{.SERIES. 


The  following  table  gives  the  averaged  results  obtained  thus  far  in  this 
investigation. 

TABLE  I. 

Convergence  Wave-lengths. 


Atomic 
Number. 

Element. 

K 

Series. 

L 
Series. 

M 

Series. 

N 
Series 

6 

Carbon 

4?  6  A 

375      A 

8  

Oxygen 

23.8 

248 





Aluminium 



100 

326     A. 



Silicon 



82.5 





Titanium 



24.5 

85.3 



Iron 



16.3 

54.3 

247  A. 

Copper 



12.3 

41.6 

116 

There  is  overlapping  of  these  results  with  those  obtained  by  Professor 
Millikan1  in  the  extreme  ultraviolet.  He  has  definitely  placed  the  con- 
vergence wave-length  of  the  L  series  of  carbon  at  360.5  A.  He  finds  a 
strong  iron  line  at  271.6  A.,  aluminium  lines  at  136.5  A.  and  144.0  A.  and 
an  oxygen  line  at  231  A.  These  are,  presumably,  the  M  a  iron  line, 
the  L  a  aluminium  lines  and  the  L  a  oxygen  line.  He  does  not,  however, 
find  an  aluminium  line  near  or  slightly  longer  than  326  A.  Remembering 
that  our  values  refer  to  convergence  wave-lengths,  the  agreement  seems 
to  be  good.  It  must  be  remembered  that  the  accuracy  of  the  present 
method  is  relatively  poor  at  the  longest  wave-lengths,  owing  to  the 
weakness  of  the  radiation  and  the  uncertainties  introduced  by  the 
distribution  of  velocities  of  the  bombarding  electrons.  This  correction, 
which  could  not  amount  to  more  than  two  or  three  volts,  was  entirely 
neglected  since  the  potential  drop  across  the  filament  was  about  sufficient 
to  balance  the  average  kinetic  energy  of  emission  of  the  electrons.  At 
the  higher  voltages  used,  this  correction  would  be  entirely  negligible, 
but  it  may  have  introduced  small  errors  at  the  lowest  voltages. 

It  is  proposed  to  continue  this  investigation  with  other  metals.  A 
comparison  of  complete  data  given  by  this  method  with  those  being 
obtained  for  radiation  from  gases  by  Foote  and  Mohler  should  prove 
extremely  interesting  and  might  lead  to  some  explanation  of  the  condition 
of  electrons  in  atoms  of  solids  and  of  the  failure  of  Kossel's  relation  when 
applied  to  characteristic  radiation  from  solids. 

I  take  this  occasion  to  express  my  indebtedness  to  Professor  Karl 
T.  Compton  for  his  inspiring  interest  in  this  work  and  for  the  kindly 
help  which  he  has  given  all  through  the  experiments. 

PALMER  PHYSICAL  LABORATORY, 

PRINCETON  UNIVERSITY. 
1  Astrophys.  Jour.,  52,  p.  47,  1920,  and  private  correspondence. 


/ft 


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